The present invention relates generally to a technique for investigating characteristics of an acoustic wave propagating through an acoustic field, and more particularly to a method and an apparatus for forming a photographic record representing acoustic characteristics such as distribution of sound pressure, spread of sound wave, and distribution of equiphase surface, of noisy sound generated from a noise source such as a rotating tire an in general, machines.
In order to take a photograph of a distribution of sound pressure in a sound field, there have been proposed various methods. In one of the known methods, a plane in the sound field is scanned continuously by means of a microphone secured to a scanning device to which is also arranged a light source such as a light emitting diode.
An output signal from the microphone is amplified and detected, and then a switching circuit is controlled by an output signal from the detector in such a manner that when the output signal exceeds a predetermined level corresponding to a given minimum sound pressure to be detected, the switching circuit is made conductive to turn on the light emitting diode to form a locus of light spot. An image of the locus of light spot thus formed is then recorded on a photographic film by means of a still camera whose shutter is remained opened during the scanning.
In such a known method of forming a photographic record of the sound field, the image formed on the film has no sharp boundary between bright and dark areas due to a hysterisis characteristic of the switching circuit. As shown in FIG. 1, when an output signal M from the detector exceeds a turn-on voltage level E.sub.ON, the switching circuit is made conductive to ignite the light emitting diode. When the output signal M decreases below a turn-off voltage level E.sub.OFF, the switching circuit is made non-conductive to turn-off the light source. Usually, the turn-on voltage level E.sub.ON is different from the turn-off voltage level E.sub.OFF. Therefore, when the microphone and light emitting diode are reciprocally scanned, the light emitting diode is turned on or turned off during the backward scanning at positions slightly displaced by a distance d from positions at which the light emitting diode is turned off or turned on during the forward scanning, even if the output signal M from the microphone is not changed between the forward and backward scannings. In this manner, a boundary of the image recorded on the film lacks the sharpness. The above explained drawback may be obviated by scanning the sound field always in the same direction. But, in such a case, it is apparent that the scanning period would be prolonged to a great extent.
Further, in the known method, the scanning member for supporting the microphone and light source is moved continuously to effect a so-called continuous move system. In this case, the influence of noise contained in the output signal M of the microphone directly appears on the final photographic film, because the light intensity of light spot locus recorded on the film does not represent an average of instataneous sound pressures at respective scanning points, but represents instantaneous sound pressures at respective scanning points. Therefore, when noises n are superimposed on the output signal M, the light source is erroneously lighted on and the distribution of sound pressure could not be recorded accurately.
In order to obviate the above mentioned drawback, it may be considered that the microphone and light source are moved intermittently and sound pressures detected at respective points are averaged to remove the influence of noise. In this case, it will be very difficult to control a driving mechanism of the scanning device and a very long scanning time is required. Further, a light spot locus recorded on the film would not be continuous and thus, a sharp image could not be obtained. This drawback may be eliminated by reducing a distance between successive stop positions of the scanning device. However, in this case, the control of the scanning mechanism will be more difficult and the scanning will require much longer time.